Transformer core



Patented May 28, 1946 TRANSFORMER CORE Clifford C; Horstman and John H.Bramble,

' Sharpsville, Pa., assignors to Westinghouse Electric Corporation, EastPittsburgh, Pa., a corporation of Pennsylvania Application March 3,1945, Serial No. 580,862

6 Claims.

Our invention relates to magnetic core structures for electricalapparatus such as transformers, and, particularly, to core structureshaving three winding legs for accommodating the several phase windingsof a three-phase electrical apparatus.

Our invention relates, particularly, to core structures for electricaltransformers formed of layers of magnetic sheet steel having preferredmagnetic characteristics in the direction of the sheet in which the coreparts are formed, the layers of steel sheets or laminations being bondedtogether into solid brick-like sections having butt joints at theiradjoining faces.

It has been found that if a core is formed of layers of sheet steelhaving the most favorable magnetic characteristics in the direction inwhich the magnetic flux passes through the strip or sheet of steel, acore having high permeability and low watts loss will result.

The magnetic material comprising the core may be formed into magneticcircuit loops by winding a continuous strip of material flatwise to givethe desired dimensions, the loop being cut to provide butt joints atsuitable places to form separable core sections that fit over the coppercircuit conductors comprising the windings of the transformer. Thesebutt joints may be located in the winding leg portions of the core oradjacent the ends of the Winding leg portions.

It has been found desirable to Work the meeting faces between theseparable core sections in order to provide smooth fiat surfaces at thebutt joint between them and to provide a minimum air gap. It has alsobeen found desirable to insulate the magnetic material between theseparable sections of the core meeting at the butt joints. Materialsuitable for this purpose may be one of the combination of long-chainmolecules of the alvar or vinylite type or other oil and acid-resistingresins, such as described in U. S. Patent 2,318,095, issued May 4, 1943,to H. V. Putman for Core structures, and assigned-to the same assigneeas this application. These materials are somewhat thermoplastic and havethe property of tenaciously adhering to the metal surfaces. It isnecessary to bring'the core faces together under pressure to reduce thethermoplastic material tothe desired predetermined thinness.

It has been found that transformer cores formed of wound loops ofmagnetic material cut to provide butt joints as above described aresensitive to changes in pressure, and if the pressure is not appliedperpendicularly to the face of.

(Cl. F15- 356) the joint, the watts loss of the core increases somewhatwith increase in pressure. For example, if the core sections are clampedtogether in such manner that the surfaces of the butt joints are notparallel, localized strains will be set up in the core structure whichwill increase the loss. It has been found that the best results areobtained by providing pressure at the joint perpendicularly to theworked faces of the laminations such as is supplied by metal bandsapplied as herein disclosed. When the separable sections of such coresare held together by means of metal bands, the bands provide for drawingthe worked faces toward each other to reduce the thickness of thethermoplastic material between them. These bands are elastic and areapplied under sufficient tension and elongation to maintain the desiredpressure after the worked faces are drawn together, and in spite of theslight contraction of the band that may be caused by any flowing of theinsulating material in the butt joint due to its plastic nature.

The Putman patent referred to above shows loops of magnetic materialemployed for singlephase transformers in which there are only two corelegs having butt joints to be held under pressure.

It is an object of this invention to provide a three-phase corestructure of the above-indicated character having three parallel windingleg portions involving butt joints associated with each leg portion inwhich the core parts are held together by means of metal bands that aresuiciently elastic and are sufficiently elongated as to provide acontinuous force on the parts of the core in spite of shrinkage of theoveral1 dimensions thereof.`

It is another object of the invention to provide means particularlyadapted to secure a continuous pressure on the middle one of the threecore legs of a three-phase core structure in which the three legs are inalignment in order to insure substantially the same pressure on the buttjoints in each ofthe three leg portions of the core structure.

kOther objects and advantages of the invention will be apparent from thefollowing description of preferred embodiments thereof, reference beinghad to the accompanying drawing, in which:

Figure 1 is a side elevational View of a single core loop in which ametal band is applied in accordance with prior art practice;

Fig. 2 is a side elevational view of a threephase core having threewinding legs in which a metal band is applied following the mannerillustrated in Fig. 1;

, Fig. 3 is a side elevational view of a three-phase core in which meansis provided in accordance with one embodiment of the` invention formaintaining substantially constant pressure on the faces of thebuttjoints associated with each of the three winding legs of the corestructure;

Fig; 4 is a side elevational view of a portion of a core structureillustrating another embodiment of the invention;`

Fig. 5 is a top plan view of a core corresponding to Fig. 4 and in whichportions thereof are broken away; l"

Fig. 6 isa side elevational view of a three-phase core structureorganized in accordance with an other embodiment of the invention;

Fig. 'I is a ide elevational view of a portion of the core s ructure`illustrating another embodiment of the invention;

Fig. 'la is a. detail; and

Pig. 8 is a side elevational view of a core structure illustrating afurther embodiment of the invention. l

Referring to Fig. 1, a single core loop magnetic structure I isillustrated consisting of a-plurality of layers ofiturns of a strip 2 ofmagnetic material wound` continuously to provide a plurality of)animations or sheets of steel outlining a substantially, rectangularwindow 3. After the loop has beenlwound tolprovide the necessary numberof turns and d several turns bonded together as hereinafter pointe out,the core loop is cut along the line 4 to provde upper and lower loopsections ll and 3. each forming a U-shaped member with the laminationsbent'i'latwise at the bends of the U, thecut ends presenting s.plurality of faces at the butt joint surfaces 4. Either during tinewinding operation or thereafter, prior to cutting along the lines 4, thelayers of magnetic material are bonded together by an insulating andbonding material adhering to the opposite sides of each of the severalturns or laminations comprising the core and heat-treated so that thegroup of laminations are yieldably but rigidly at tached so that whencut along the line 4, solid U-shaped pieces result which maybereassembled about preformed coil structures comprising the copperconductors of the transformer windings. The particular treatment maycorrespond to that described in Patent No. 2,293,951, filed by J. B.Seastone and C. C. Horstman, for Induction apparatus and method of coreconstruction therefor, issued August 25, 1942, and assigned to the sameassignee as this application. The core loop shown in Fig. 1 whenreassembled about the copper conductors of the transformer winding isheld with the faces of the two U-shaped core sections under pressure atthe surfaces 4 by applying a steel band I about the coe loop, andattaching a clip 3 locking the ends of the steel band l together whilethe band is retained under a predetermined tension so as to maintain theband sumciently elongated as to maintain the desired pressure on thebutt joint surfaces 4.

Fig. 2 illustrates a core structure comprising three core loops II, I2,and I3 formed in the manner above described with respect to Fig. 1, withthe two smaller loops I I and I2 surrounded by the larger loop I3. Thetwo smallerloops II and vI2 are formed of an equal number of turns ofthe strip of magnetic material and then surrounded by the turns of theloop I3, also having a number of turns equal to those in the loops IIand I2. The three core loops in a unit are annealed and impregnated in amanner described for Fig. 1 and are then cut at three places indicatedby the numeral I4 along the same plane to provide upper and lower coresections. The several surfaces formed by cutting along the lines I4 areworked in the same manner as described for the structure shown in Fig. 1to provide cooperating accurately smooth surfaces engagingcorrespondingly accurately smooth surfaces to form the butt joints inthe three core legs. When the core structure is cut along the line I4,the loop II is separated into upper and lower sections I5 and IB,respectively, the loop I2 into upper and lower sections I1 and I3,respectively, and the loop I3 into upper and lower sections I9 and 20,respectively. When the core structure is reassembled about thetransformer windings, a steel band 2l similar to the band 1 in Fig. 1passes about the outer one I3 of the three magnetic loop structures andis held together by a clip 22, the band being applied under tension asdescribed with respect to Fig. l. When reassembled, the structureprovides three winding legs 23, 24, and 25 for accommodating thewindingsof the three phases, respectively, of the apparatus, the winding leg 23including the left vertical portions of the loops I3 and II, the windingleg 24 including the two adjacent vertical portions of the loops I I andI2, and the winding leg 25 including the two right portions of the loopsI2 and I3. The winding leg portions of the core structure are connectedby yoke portions comprising the arched-shaped end members of the threeloops, the three upper arches being shown at 26, 21, and 28, and thethree lower archedshaped yoke members being shown at 29, 30, and 3I.

In the structure shown in Fig. 2, the force applied by the steel band 2|is so applied that the vertical components that are available formaintaining pressure on the surfaces on the opposite sides of the buttjoints I4 do not maintain sufficient pressure on the middle leg 24.

We have provided means for increasing the pressure on the butt joint ofthe middle leg above that existing in the structure shown in Fig. 2. Inthe embodiment of the invention shown in Fig. 3, blocks 35 and 36,`which may be of wood or other suitable material, are positioned betweenthe central part of the upper and lower yoke portions of the core andthe steel band 2I surrounding the core, so as to hold the band 2I archedabove and below the upper and lower yoke portions of the core to providea vertical component of force which is communicated through the blocks35 and 36, respectively, and through the yoke portions of the corestructure to the surfaces on thc opposite sides of the butt joint I4 inthe middle winding leg 24. By providing blocks of the proper shapegiving the necessary spacing of the band 2| from the yoke of the core,it is possible to apply an equal force to all three butt joints in thethree core legs. The arrows on the outside of the band 1 in Fig. 1 andon the outside of the band 2i shown in Fig. 3, indicate the generaldirection of the force applied to the yoke portions of the core.

Fig. 4 illustrates a portion of a core structure similar to that shownin Fig. 2 in which the central winding leg 24 is formed by the adjacentparts of two magnetic loops II and I2 in the same manner as in Fig. 2,and the core structure is cut along the line I4. For the purpose ofapplying the desired force to the opposite surfaces of the butt joint I4in the embodiment of the invention shown in Figs. 4 and 5, a wedgeshapedblock 4I is provided having a trapezoidal cross-sectional area adaptedto engage the outer curved portions 42 and 43, respectively, of thewinding loops II and I2. The block 4I may have a length correspondingsubstantially to the width of themagnetic sheet steel employed to formthe core loop structures and is provided with a depression therein alongthe outer surface providing a seat to accommodate the steel band 44which is similar in general character to the steel band 2I applied aboutthe three core loops. It will be understood that a similar block 4I isprovided at the lower end of the middle winding leg 24, and that theband 44 is positioned in the two blocks so that when drawn togetherunder a predetermined force and attached by a clip 45, as shown in Fig.5, a predetermined force will be applied from the steel band 44 throughthe blocks 4I to the adjacent parts of the core loops I I and I2 tomaintain a predetermined desired pressure on the opposite faces of thebutt joint I4.

Fig. 6 illustrates another embodiment of the invention generally similarto that shown in Figs. 4 and 5 in which the seats provided for receivingpressure from the steel band 44 are shown as the surfaces of strips ofsteel 5I and 52. The strip 5I is welded securely to the U-shapedportions I5 and I1 of the loops II and I2, respectively, and the strip52 is welded to the U-shaped sections I6 and I3. The steel band 44passes between the steel strip 5I, and the upper U-shaped section I9ofthe core loop I3 and between the lower strip 52 and the U-shapedsection 20.

Fig. 7 is a side elevational View of a portion of the core structuregenerally similar to Fig. 4, illustrating a method of applying the band44 about the entire core structure. A trapezoidal steel pressure element5I is shown welded to a strip of steel 52 to `apply pressure to thecurved portions of the U-shaped sections I5 and II, respectively, of thecore loops II and I2. In this embodiment of the invention, a block 53 ismounted above the U-shaped section I9 of the outer core loop foraccommodating the band 44, and may be rounded at the ends, as shown inFig. 7a, to more readily accommodate the band 44. It will be appreciatedthat the structure shown at the upper end of the winding leg 24 in Fig.7 is duplicated at the lower end, and that the band 44 applies pressureto two blocks 53 at the upper and lower central portions of the outercore loop I3 which pressure is communicated through the outer loop totwo pressure members 5I at the upper and lower ends of the two loops I Iand I2, respectively, to directly apply pressure thereto on oppositesides of the butt joint I4.

In the embodiment of the invention shown in Fig. 8, pressure is appliedto the central winding leg 24 in a manner generally similar to thatillustrated in Fig. 3 by spacing the band 2| along the central part ofthe yoke portion of the core so as to apply a vertical component offorce opposite the ends of the middle winding leg that does not existwhere the band 2| is flat or straight as in Fig. 2. This is accomplishedin the structure shown in Fig. 8 by providing a series of steel strips6| between the successive turns of the sheet steel forming the outerloop I3 of the core structure. These sheets of steel 6I are of differentlengths, the longer sheets being closer to the smaller core loops I Iand I2, and the larger ones further away so as to space the steel band2I a greater amount directly opposite the ends of the middle winding leg24 to provide a greater vertical component of force from the steel band2I to the core structure that is communicated to the opposite faces ofthe butt joint I4 of the middle leg 24.

It will be noted that in the embodiments of the invention illustrated inFigs. 3 and 8, the force applied to the opposite surfaces of the buttrjoint I4 in the middle winding leg 24 is applied directly from the band2| through the yoke portions of the outer loop of the magnetic circuitI3. In the embodiments of the invention illustrated in Figs. 4, 5, 6,and 7, an additional steel band 44 is employed. In Fig. '7, this bandalso applies pressure to the outer surfaces of the yoke portions 28 and29 of the outer core loop I3 which is applied therefrom to the innercore loops I I and I2 and to the butt joint in the middle winding leg24. In the embodiment of the invention shown in Figs. 4 and 5 and alsoin Fig. 7, the force is applied directly from the steel band 44 to theopposite ends of the inner core loop members II and I2 directly adjacentthe portions of these two loops forming the middle winding leg'24.

rIt will be apparent to those skilled in the art that modifications ofthe subject matter described and illustrated within the spirit of ourinvention may be made without departing from the invention, and we donot wish to be otherwise limited than by the scope of the appendedclaims.

We claim as our invention:

1. In a three-phase core structure for electrical apparatus, incombination, core parts comprising three leg portions positioned in arow in parallel relation and connecting yoke portions connecting the legportions, the core structure comprising separable core parts formed oflaminations of magnetic sheet material united by bonding materialapplied to the laminations, the separable core parts having meetingfaces worked thereon for providing butt joints in the several legportions of the core, the yoke structure at each end of the leg portionsof the core comprising three arch-shaped portions adjacent the oppositeends of the leg portions, two of the three arch-shaped portions being oflesser span than the third, the two arch-shaped portions of lesser spanconnecting the middle leg portion with the two outer leg portions,respectively, and the archshaped portion of greater span connecting thetwo outer leg portions of the core, a resilient band of metal undertension about the core structure for maintaining the core parts inposition and for maintaining a substantially constant pressure on thebutt joints, and additional means for applying pressure on the buttjoint in the middle one of the three winding leg core portions.

2. In a three-phase core structure for electrical apparatus having threewinding legs arranged in a row with their axes lying in the same planeand parallel to each other, the core structure comprising a plurality ofcore loops each formed of magnetic sheet material wound ilatwise, thespaces between the successive layers of magnetic sheet material beingfilled with a substantially non-flowable, non-volatile, molded, solidmaterial for solidly supporting the several layers of material againstrelative movement, each loop being cut to form closely fitting buttjoints on opposite sides thereof and to provide separable core sections,the core structure including three core loops, two of the core loops'having side portions adjacent each other together forming the centralone of the three winding legs and having side portions remote from eachother forming parts of the two outer winding legs, respectively, thethird core loop extending about and enclosing the other two andincluding parts of the two outer winding legs, a resilient band of metalunder tension about the outer loop of the core structure for maintaininga substantially constant pressure on the butt joints, and additionalmeans for applying pressure on the butt joint in the middle one of thethree winding legs.

3. In a three-phase core structure for electrical apparatus having threewinding legs arranged in a row with their axes lying in the same planeand parallel to each other, the core structure comprising a plurality ofcore loops each formed of magnetic sheet material wound flatwise withintervening bonding layers of an adherent insulating bond betweenadjacent laminations solidly holding the laminations together, each loopbeing cut to form closely fitting butt joints on opposite sides thereofand to provide separable core sections, the core structure includingthree separate core loops including, respectively, the first and secondwinding leg, the second and third winding leg, and the first and thirdwinding',` legthe third-named loop extending about and enclosing theother two loops, a resilient band of .metal under tension about theouter loop of the core structure for maintaining a substantiallylconstant pressure on the butt joints, and additional means for applyingpressure on the butt joint in the middle one of the three `winding legs.

4. In a. three-phase core structure for electrical apparatus. incombination, core parts comprising three leg portions positioned in arow in parallel relation and connecting yoke portion-s connecting theleg portions, the core structure comprising separable core parts formedor laminationg of magnetic sheet material united by bonding materialapplied to the laminations, the separable core parts having meetingfaces worked thereon for providing butt joints in the several legportions of the core, the yoke structure at each end of the leg portionsof the core comprising three arch-shaped portions adjacent the oppositeends of the leg portions, two of the three arch-shaped portions being oflesser span than the third, the two arch-shaped portions of lesser spanconnecting the middle leg portion with the two outer leg portions,respectively, and the arch-shaped portion of greater span connecting thetwo outer leg portions of the core, a resilient band of metal undertension about the core structure for maintaining the core parts inposition and for maintaining 'a substantially constant pressure on thebutt joints, and additional means for applying pressure on the buttjoint in the middle one of the three winding leg core por tions, saidmeans comprising a seat at each end of the middle winding leg portionsadjacent the two arch-shaped portions of lesser span and within thearch-shaped portion of greater span and a resilient metal bandpositioned in said seats at the opposite ends of themiddle winding legunder tension about the middle winding leg for maintaining constantpressure on the butt joint in thatleg.

5, In a three-phase core structure for electrical apparatus having threewinding legs arranged in a row with their axes lying in the same planeand parallel to each other, the core structure comprising a plurality ofcore loops each formed of magnetic sheet material wound flatwise, thespaces between the successive layers of magnetic sheet material beinglled with a substantially non-flowable, non-volatile, molded. solidmaterial for solidly supporting the several layers oi' material againstrelative movement, each loop being cut to form closely fitting buttjoints on opposite sides thereof and to provide separable core sections,the core structure including three core loops, two of the core loopshaving side portions adjacent eacli other together forming the centralone of the three winding legs and having side portions remote from eachother forming parts of the two outer winding legs, respectively, thethird core loop extending about and enclosing the other two andincluding parts of the two outer winding legs, a resilient band of metalunder tension about the outer loop of the core structure for maintaininga substantially con stant pressure on the butt joints, and additionalmeans for applying pressure on the butt joint in the middle one of thethree winding legs, said additional means comprising a seat at each endof the middle winding leg and a resilient metal band under tensionpositioned in said seats and extending about the leg along the oppositesides thereof.

6. In a three-phase core structure for electrical apparatus having threewinding legs arranged in a row with their axes lying in the same planeand parallel to each other, the core structure comprising a plurality oicore loops each formed of magnetic sheet material wound flatwise withintervening bonding layers of an adherent insulating bond betweenadjacent laminations solidly holding the laminations together, each loopbeing'cut to form closely fitting butt joints on opposite sides thereofand to provide separable core sections, the core structure includingthree separate core loops including,' respectively, thx: rst and secondwinding leg, the second and third winding leg, and the first and thirdwinding les. the third-named loop extending about and enclosing theother two loops, a resilient band of metal under tension about the outerloop of the core structure for maintaining a substantially constantpressure on the butt joint-s, and addi tional means for applyingpressure on the butt. joint in the middle one of the three winding legs.said additional means comprising a seat at cach end of the middlewinding leg and a resilient metal band under tension positioned in saidseats and extending about the leg along the opposite sides thereof.

CLIFFORD C. HORSTMAN. JOHN H. BRAMBLE.

